As a technique to measure the flow rate of a fluid in a flow path, there has been known a technique to measure the propagation time of an ultrasonic wave in the downstream direction of the flow path and the propagation time of the ultrasonic wave in the upstream direction of the flow path to calculate a flow rate on the basis of the difference between these two propagation times. In this technique, flow rates can be precisely measured using the difference in propagation time even if the propagation velocity of the ultrasonic wave varies due to temperature change or the like.
Incidentally, flow rates are required to be frequently measured in such applications as water meters where a fluid flow rate dynamically changes, in order to accurately know the dynamically-changing flow rate. If flow rates are frequently measured, there is concern over an increase in the power consumption of a measurement device.